The use of optical fiber communication systems has increased significantly during the last few years. It appears likely that the use of this mode of communications will continue to increase in the future. Accordingly, companies involved in this type of communications have sought ways not only to improve the manufacture of optical fibers and cables, but also techniques for providing connections between fiber lengths.
Establishing connections between optical fibers is not a simple task. Typically, an optical fiber includes a core which may be as low as about 8 microns in diameter and a cladding having an outer diameter of about 125 microns. The optical fiber is coated with a material which has an outer diameter of about 250 microns.
Various devices and methods have been developed for connecting two optical fibers. Connectors and splices are two general categories of optical fiber joining techniques. Whereas connectors relate to devices which are capable of repeated connections and disconnections, splices are usually used when a relatively low loss is desired in connecting two optical fibers with the probability of disconnection and reconnection being relatively low.
Currently, there are two general categories of optical fiber splices-fusion splices and mechanical splices. In fusion splices, the ends of two optical fibers are brought together and melted by a flame or electric arc, for example, in order to join the ends. Although the fiber cores may be aligned prior to making the fusion splice, the fusion process may disturb that alignment and cause undesirable splice losses. In mechanical splices, the optical fiber ends are brought together and joined by mechanical means or by an adhesive material. Those splices which are made with an adhesive material are referred to as bonded splices.
Splices are made in both multimode and single mode lightguide fibers. The low loss and high bandwidth of single mode optical fibers promise excellent high capacity long distance communications. However, the relatively small core diameter of single mode fiber makes splicing more difficult than with multimode fibers, and the effects of end quality and transverse and angular misalignment are more critical. Reports of fiber losses as low as 0.35 dB/km at 1.3 .mu.m, and even less at 1.55 .mu.m, make low loss splicing techniques important for maximum repeater spacing. For example, if a splice is placed, on the average, every kilometer in a fiber that has an inherent loss of 0.4 dB/km, and if the splice itself adds an additional 0.2 dB loss, the average loss of the spliced fiber will be 0.6 dB/km. However, if the splice loss is reduced to 0.1 dB, the average loss of the spliced fiber would be 0.5 db/km. For a typical single mode optical fiber system, this reduction in loss is estimated to yield an increase in maximum repeater spacing of about 1 to 2 kilometers. Thus, a very significant economic benefit is realized by reducing the splice loss between fibers.
Conventional splicing techniques that rely on the alignment of the outer surface of the fiber cladding achieve relatively low splice losses only for fibers with well-centered cores having an eccentricity of less than 0.5 micron and well-controlled outer diameters . Submicron core centering tolerances cannot always be maintained in large-scale manufacturing; therefore, the splicing of non-identical fibers relying on cladding alignment methods are expected to result in higher losses.
In one technique, two optical fibers having end faces that are substantially flat and substantially perpendicular to the axes of the fibers are placed end-to-end. Next, a slotted tube is placed to surround the ends, and then at least partially filled with an adhesive material, typically ultraviolet (UV) curable adhesive material. The fibers are aligned to produce minimum scattering of radiation directed through the fibers, as measured by a scattering detector, and the adhesive material is cured. Optionally, a sleeve, typically in the form of glass tubing, is then moved over the splice, seated with adhesive material, and cured.
Although the last-mentioned technique has yielded losses typically less than 0.1 dB, the splices may experience a problem caused by the environment. Temperature cycling causes the tube and/or the sleeve to expand and contract. As a result, thermally induced stresses may be imparted by the tube to the fibers and hence to the splice. This may lead to failure of some splices, which would result in interrupted circuits and require repair.
Seemingly, the prior art has not yet dealt with this problem. Nonetheless, it is a problem that must be addressed in order to provide lightguide systems, particularly single mode systems, with the reliability that users have come to expect in the communications industry.